Oscillation generator



Feb. .23, 1.937. p, FARNSWQRTH 2,071,516

I OSCILLATIIQN GENERATOR I Filed July 5, 1934 .2 Sheets-Sheet 1 w l.OUTPUT OUTPUT IN VEN TOR. 199/107." FAB Kim 19/5 A TTORNE YS.

Feb. 23, 1937. P. T. FARNSWORTH 2,071,516

050 ILLATION GENERATOR Filed July 5, 1934 2 Sheets-Sheet 2 I N V E N TORPH/LO 7." PAPA SW02? J31 J7 M Patented Feb. 23, 1931 UNITED STATESPATENT OFFICE Philo T. Farnsworth, San Francisco, Calif., as-

signor to Farnsworth Television Incorporated, a corporation ofCalifornia Application July 5, 1934, Serial No. 733,837

27 Claims. (Cl. 250-36) This invention relates to electronicoscillators, and particularly to oscillators wherein the elect"on flowis derived, substantially in its entirety, from so-called secondaryelectrons liberated from solid conductors by bombardment.

The primary object of this invention is to provide a new type ofelectronic oscillator. Among the other objects are: To provide anoscillator which will convert direct current energy at extremely highefiiciencies; to provide an oscillator which is substantially free fromfrequency limitations, and is particularly adapted to the generation ofoscillations at frequencies above 30 megacycles; to provide a type of.oscillator which is equally well adapted for the generation of eithervery small or very large amounts of power; to provide an oscillatorwherein the electrical circuits aresymmetrical, so that the neutralpoint of the circuit may be grounded without introducing difiiculties orcomplications; to provide an oscillator of the high vacuum type, whichis independent of ionization phenomena andis not subject to theinstabilities and inconsistencies which such phenomena introduce; toprovide an oscillator which is easily triggered off; and to provide anoscillator having no heated electrode of the thermionic type, and whichis correspondingly free of the difficulties and complications introducedby cathode heating circuits.

Other objects of my invention will be apparent or will be specificallypointed out in the description forming a part of this specification, butI do not limit myself to the embodiment of the I invention hereindescribed, as various forms may be adopted within the scope of theclaims.

Referring to the drawings:

Figure 1 is a schematic diagram showing one form of the oscillator of myinvention, including the oscillating tube and its associated circuits.

Figure 2 is a diagram showing a modification of the form of oscillatorshown in Figure l, wherein the amplitude of oscillation isself-limiting.

Figure 3 is a cross sectional View showing an oscillator tube positionedwithin a permanent magnet guiding-field structure.

Figure 4 is a cross sectional View of a modified term of the tube whichwill generate oscillations without the use of a guiding or focusingfield.

Figure 5 is a circuit diagram illustrating a tube of the type shown inFigure 4 with a diiierent type of oscillating circuit from that shown inFigure 1.

Figure 6 is a schematic diagram showing a modified form or" my inventionwherein an asymmetrical tube and circuit are employed.

light in the visible spectrum.

pointed out the regenerative nature of the multi- Figure '7 is a graphshowing the characteristic form of a secondary emission curve.

In my copending application, Serial Number 692,585, filed October '7,1933, I have described an electron multiplying device wherein, byutilizing 6 the phenomena of secondary electron emission, extremelysmall currents may be multiplied to give very large final outputs. Theinitial current necessary in this apparatus may be so small as to beimmeasurable by any ordinary method, e. g., the photoelectric currentemitted from nickel by I have there plication, stating that by backcoupling the output of this device into its input, self-sustainingoscillations can be produced. This application specifically relates tothe production of such selfsustaim'ng oscillations by the method thereinindicated.

Considered in its broad aspect, the essential features of the inventioncomprise a pair of electrodes defining an electron path. A cloud ofelectrons oscillated along this path by potentials applied to theelectrodes, and striking one of them, which may be termed cathode, withsufficient velocity to release secondary electrons at a ratio greaterthan unity as compared with the impacting primary electrons, causes acurrent flow from a d-c source of potential included in a circuit withthe electrodes. A potential drop due to this current is applied in phaseto apply the necessary impacting velocity to the oscillating cloud ofelectrons, and thus the oscillation becomes self-sustaining andrelatively large amounts of oscillating power may be withdrawn from thecircuit. The energy for maintaining the oscillation is, of course,derived from the (1-0 source, which must be sufiiciently high in potenvtial to release the secondary electrons at the required ratio.

For the oscillations to be self-starting as well as self-sustaining, therequirements are somewhat more rigorous. If an exciting oscillation ofsufficiently high voltage be applied initially, there will be enoughcasual electrons present in the spacee between the electrodes to buildup the secondary emission currents of the values re uired to produce thenecessary voltage drop with'almost any electrode material, provided theelectrons are 5g sufi ciently accurately guided along their path so thatthere is little probability of their being collected before they strikethe cathode. In order that the oscillations be self-starting, however,the work function of the cathode surface should be 5 as low as possible,and for this reason it 'is preferred to provide the cathodes withsurfaces of photoelectric material, suchas the alkali metals, potassiumhydride, or the extremely sensitive caesium silver oxide surfaces whichhave proved the best of those that have yet been tried in the laboratoryfor this purpose. With such a surface it has been found that almost anyshock to the circuit will start the operation, even under the mostunfavorable conditions. After the device has been operating a time andthe electrodes become warm, it becomes more difficult to preventoscillations than it is to start them.

Considering the invention now in greater detail, Figure 1 shows a formof my invention which is one of the simplest and most readilyconstructed and operated. The vacuum tube which forms the heart of theapparatus is of the type described in my prior application abovereferred to, and comprises an evacuated envelope I of cylindrical form,having cathodes 2, 2' mounted in the opposite ends thereof. The cathodesare supported by lead wires 4, which are sealed through the walls of theenvelope, and comprise discs of pure silver whose surfaces have beenoxidized and coated with caesium in a manner well known for theformation of photoelectric cells. An anode 5, comprising a ring fittingsnugly the walls of the tube is mounted midway between the two cathodes,connection being made to the anode through a lead 6 sealed through thewall of the tube.

Connected between the two cathodes is an oscillating circuit comprisingan inductor 1 in parallel with a variable condenser 9. A central tap illon the inductor connects through a potential source H to the anode,preferably through radio frequency chokes l2 and I4, which, although notstrictly necessary, improve the performance of the apparatus andincrease its oscillating output. A secondary coil I5, coupled to theinductor 1 maybe used to withdraw power from the circuit, this method ofcoupling being shown as typical of many well known methods ofwithdrawing energy from any oscillating circuit.

- tion of the electrons from the cathode 2 will cause.

Surrounding the tube is a solenoid l6, which is supplied with directcurrent from a source II, the current being accurately adjustable bymeans of the rheostat l9.

Consider the circuit in the non-oscillatory state,

and that electrons are released from approximately the center of thecathode 2 with zero velocity. Such electrons would be accelerated towardthe anode 5, and, in the absence of the focusing field due to thesolenoid I6, would probably strike the anode and be removed from theinter-electrode space. If, however, the focusing coll be excited to theproper value, its field will guide the electrons past the anode to apoint on the opposite cathode 2' corresponding to that on the cathode 2from which they were originally liberated. During their flight they areaccelerating up to the time they reach the median plane of the anode,while from this point on they are decelerated by the electrostatic fieldsupplied by the source H, and, under the conditions mentioned, willarrive at the opposite cathode with zero velocity, having occupied inflight a time determined by the potential of the source I I and thedistance between the two cathodes.

If the resonant circuit comprising coil 1 and variable condenser 9 betuned to a frequency whose half period is approximately equal to thetime of flight of the electrons, the original liberaa current to flowfrom the source through onehalf of the inductor I to the cathode 2. Thisprovides a potential drop which appears on the two cathodes in suchphase as to accelerate the flight of the electrons causing them toimpact the second cathode 2' with a finite velocity. If this velocity besufiicient to release electrons from the second cathode at a ratiogreater than unity, the difference between the number of impactingprimary electrons and emitted secondary electrons forms a current whichis supplied to the other branch of the inductor 1, causing a voltagedrop in opposite phase, which accelerates the space current through thetube in the opposite direction. The secondary electrons emittedtherefore impact the first cathode in increased numbers and withincreased velocity, and the oscillation quickly builds up to a pointwhere it is limited either by space charge effects within the tube, byloss of energy to the output circuit, or by some other extraneouslyintroduced factor.

The factors which determine how readily such an oscillation will startare the dimensions of the tube, the-voltage of the source H, thefocusing current, and the material of the-cathode surface. Thedimensions and the voltage determine the gradient between the twocathodes. The velocity with which any electron strikes the opposingcathode is determined almost entirely by the integrated efl'ect of theoscillating potential during its time of flight. It will, therefore,have maximum velocity of impact if its entire flight is accomplishedduring one- -half cycle of oscillating potential, 1. e., during aportion of the cycle wherein the potential of the cathode from which itwas emitted is always negative with respect to the opposed cathode.Whether this be also the condition for maximum emission upon impact, ornot, depends upon the portion of the secondary emission curve upon whichthe tube is operating. The curves of nearly all materials are ofsubstantially the shape shown in Figure 7, but the coordinate valuesvary greatly for different materials. If the voltages are such thatimpact occurs with a velocity corresponding to the ascending branch ofthe curve, an increase in velocity will result in a correspondingincrease in number of secondary electrons emitted at each impact. If,however, the voltage corresponds to the crest of the curve there will beno increase in the number of secondaries per impact with incieasedvoltage, while if the voltage increases to the point where thedescending branch of the curve is in use, an increased potential willresult in a decreased number of secondaries.

The effect of the focusing field upon the oscillation is dependent uponthe number of electrons which it permits to be collected by the anode.When oscillations are first started all of the available electrons arenecessary to create the new secondaries, and therefore, under startingconditions, the focus should be quite critically adjusted. Whenequilibrium is reached, however, it is obvious that all electrons inexcess of the number of impacting primaries must be collected in eachhalf cycle. With a very plentiful supply of primaries, and a highsecondary-primary ratio, the focusing becomes very much less critical assoon as the oscillation has started, and

the circuit will oscillate. resonant frequency of the tuned circuit is afaces upon the action of the device is obvious. With the caesiumsurfaces here described, one of the readiest ways of startingoscillation is by flashing a light upon one of the cathodes. The oxidecaesium surfaces, moreover, have an extremely low work function, andsecondary ratios as high as 4 or 5 are readily obtainable. Thisobviously leads to quick build-up of circulating current in theoscillatory circuit and consequent quick and easy starting.

To give some actual values, and thus indicate.

the order of magnitude of the circuit constants involved, a tube of thecharacter described having a spacing of 5.5 centimeters between thecathodes, was operated at frequencies varying between 30 megacycles andsomething over megacycles. The corresponding voltages of the d-c sourcerequired to give the necessary cor responding times of flight variedbetween 350 volts for the lower frequency and 800 volts for the higherone.

Under these conditions it was found that oscillation could be started byflashing a light on one cathode as mentioned above, by a sudden closingof the circuit to the source H, or even by suddenly closing the focusingfield circuit. After the circuit has been in operation for a fewmoments, permitting the cathodes to become warm, and thus reducing theeffective energy which must be derived from an impacting electron inorder to cause secondary emission, such shocks became unnecessary, andoscillation would occur without any appreciable electrical shocks tostart them.

It should be noted that although the order of the period of the tunedcircuit should be 'approximately twice the time of flight of electronsunder the d-c field component, this value is not critical. For eachvalue of accelerating voltage there is a fairly wide range of tuningover which Within this range the primary factor in determining thefrequencybf oscillation. Minor factors are the accelerating voltage andthe amplitude, since this latter factor affects the velocity of theelectrons and hence affects the mean time of flight. The value of thefocusing field, due to its effect upon amplitude, and upon the length ofthe electron path, also affects frequency to a slight extent.

Once oscillation has started in the system, it will continue to build upin amplitude until the number of electrons collected by the anode ineach half cycle is equal to the excess of secondary over primaryelectrons. If the proportion of the total electron cloud collected wereindependent of the density of the cloud, the amplitude of theoscillation might continue to build up almost indefinitely, for it willbe seen that in the device of my invention there is no fixed limit tothe electron emission as there is in the ordinary thermionic tube.

In actual operation, however, the greater the density of the cloud thegreater will be the 'proportioncollected, since the increased spacecharge with high densities repels the peripheral electrons with greaterforce and tends to drive electrons to the anode. This sets a limit tothe amplitude of oscillation, but it is very easy to construct tubeswhere this limit is so high that the tube will be destroyed before it isreached. It may therefore be necessary in certain instances to reducethe focusing current as soon as the oscillation is started, in order toprevent destruction of the tube by overheating.

If the voltage builds up to a point where the secondary emission curveis operating upon its descending branch, this also makes the amplitudeof oscillation self-limiting, the voltage increasing until theproportion of secondaries to i primaries falls to a point where theexcess of electrons is equal to the number collected. It will berecognized that the load withdrawn from the oscillating circuit also hasan effect in limiting the amplitude, in that this load acts effec tivelyto decrease the voltage drop produced in the oscillating circuit by acurrent of definite amount, and hence affects the primary-secondaryratio. With certain types of load it may be highly desirable to operatethe tube upon the descending branch of the curve, since in this case anincreased load, by decreasing the accelerating voltage,.increases thenumber of electrons emitted. This, in turn, increases the voltage drop,and causes equilibrium to reestablish itself at a point corresponding tothe changed load.

The relation between the tuning, of the resonant system to the time offlight of the electrons asdetermined by the accelerating voltage mayalso limit the amplitude of oscillation.

Analytical solution of the eifect of varying the oscillatory componentof the accelerating poten' tial upon the time of flight, and hence uponthe tuning of the resonant circuit for maximum output, is extremelydiificult, and at the date of this application for patent is onlypartially complete.

In general, the time of flight varies with the phase of emission, withthe amplitude of the oscillating potential, and with the shape of theelectrostatic fields and the portion thereof through which the electronfalls. There will, for any amplitude of oscillation, be only one or atmost two phases of emission for which the time of flight is exactlyone-half cycle, and it follows that there will usually be a differencein phase between the fiight of any group of impacting electrons and thatof the secondaries which are released thereby: This phase shift may beeither increased or decreased by an increase in amplitude ofoscillation. If

increased, it serves to set a limit to amplitude,

for if it be such that the high-est emission ratio occurs at a phasewhere theemitted electrons never reach the opposed cathode, or reach itwith insufficient velocity to cause further emission, the oscillationwill tend to die out. It is for this reason that the half period of thetuned circuit should be of the same order of magnitude as the time offlight. 'Within this limit it is easy to find a frequency whereatoscillation will start, and once started, to find the conditions ofoptimum operation. Theoretically, with the tube of Figure 1, oscillationmay occur where the' time of flight is the time of any odd number ofhalf cycles, but starting such oscillations is difficult where the timeis greater than that of one-half cycle.

a condition less favorable to oscillation and limit-.

ing output.

It is by no means necessary that a solenoid be used to provide theguiding field. Figure 3 indicates a method of mounting the tube within apermanent magnetic structure, which will produce a similar effect. Thetube 2| is mounted between pole-pieces 22, which concentrate and directthe magnetic field established by permanent magnets 24.

Electrostatic methods of guiding the electron cloud may be utilized aswell as electromagnetic ones. In the tube shown in Figures 1, 2, and 3,the electrostatic fields are strongly curved, and the electromagneticfocusing fields are necessary in order to guide the electrons betweenthe cathodes. In the tube of Figure 4, however, the lines of force inthe electrostatic fields are substantially straight within the electronpath, and it may therefore be operated without the use ofelectromagnetic focusing. This tube comprises an envelope within whichare mounted the opposed cathodes 26, preferably slightly concave,

substantially as within the tubes already described. Midway between thecathodes is a conducting band 21, fitted snugly within the walls of theenvelope and connected with a lead 29 sealed through the wall of thetube. Projecting inwardly from the center of the band is an annulardiaphragm 30, and secured across the opening in the diaphragm is a gauzeor grid 3|, preferably formed of extremely fine wire of open mesh sothat the actual area which it offers to the electron stream is extremelysmall as compared with the total area of the aperture.

With this structure the lines of force between ,the edges of the cathodeand the anode are somewhat curved, but those passing down the centerWhile the tube of Figure 4 may be utilized in the same circuit as theone first described, the fact that the anode forms an electrostaticshield also permits its use in a circuit of somewhat different type asshown in Figure 5. In this case the two cathodes are connected together,and the oscillating circuit comprising the inductor 35 and condenser 36is connected between the junction of the two cathodes and the anodethrough the accelerating potential source 31. The negative end of thesource is shown as connected to ground, as is the second terminal of theparallel resonant circuit. The resonant circuit is tuned to a frequencywhose whole period is approximately equal to the time of flight of theelectrons between the cathodes.

An electron released from either cathode will be attracted to the anodeand will require approximately one-half cycle to reach this point. Ifthe phase of the potential developed in the oscillating circuit by theelectron flow from the cathode is correct, this potential will reverseafter the electrons pass through the grid 3!, so that this potentialwill still be effective to accelerate the. electrons during theremainder of the journey down the tube and will cause the secondaryelectronsto be emitted by impact with the cathode as before. With thisform of circuit there will, in general, be two electron cloudsoscillatcult to start.

acteristic that capacity between the cathodes and' anode is less for thesame frequency of operation, and hence when especially high frequenciesare desirable it .may offer certain advantages.

Figure 6 shows a 'form of the device in which but a single secondaryemissive cathode is used. This cathode 40 is mounted in one end of theevacuatedenvelope 4|. Near the other end of the envelope is an anode 42of the same type as that described in connection with the tube of Figure4, and immediately behind this anode is positioned an auxiliary cathode44, which need not have a photo-emissive surface. A focusing coil 45 isshown as supplied by a battery 46 and rheostat 41, but it is to beunderstood that any means of guiding the electron cloud, eitherelectromagnetic or electrostatic, may be used.

A parallel oscillating circuit 49 is connected in series with thecathode 40 and anode 42 through the accelerating potential source 50. Aconnection 51 leads from the negative end of the source 50 through abiasing potential source 52 to the auxiliary cathode 44, the potentialof the source 52 being sufl'i'cient to maintain the auxiliary cathodealways negative to'the cathode 40.

In this case the action of the device is substantially the same as thatof one-half of the tube of Figure 5. Electrons released from the cathode40 reach the anode in approximately one-half cycle. Passing through theanode they are immediately reversed in direction by the field betweenthe anode and the auxiliary cathode, and return through the anode at thesame velocity which they had attained on first reaching it. By thistime, however, the oscillating potential between anode and cathode hasreversed, so that approximately one full cycle after being released fromthe cathode 40 they again impact it with a velocity due to theintegrated efiect of the oscillating voltage during their time offlight. The release of additional secondary electrons and repetition ofthe,cycle occurs as before.

The circuit of Figure 6 has the same disadvantages compared with that ofFigures 1 and 2 as does the circuit of Figure 5, with the additionaldisadvantage that a certain number of the electrons will be collectedeach time the cloud passes through the anode 42. After the oscillationhas become stabilized this is no material disadvantage, but it doesrender the oscillation more dim- On the other hand, the anodecathodecapacity which is effectively shunted across the oscillating circuit isonly one-half that of the already small capacity contributed by thestructure of Figure 5. Thus the use of this arrangement may be justifiedin certain circumstances, even though an auxiliary oscillator, laterremoved, may be necessary to start it in oscillation.

It should be recognized that the action of the oscillators heredescribed is complex, that many factors enter into the final result, andthat, owing to the interaction of these factors, a change in some onecondition may, in certain instances, produce directly. the oppositeeffect from what might generally be expected. Thus imposing a load onthe oscillator will usually change the phase as well as the magnitude ofthe potentials in the oscillating circuit, and for many conditions oftuning and accelerating voltagewill increase the intensity ofoscillation even though the tube be operating on the rising branch ofthe secondary emission curve. Similarly special conditions may be foundwhere increase of focusing field increases oscillation intensity,instead of decreasing it, as isthe usual case. Anomalies of thischaracter are probably more apt to occur as limiting conditions forselfoscillation are approached, for under these circumstances a minor orsecondary efiect, operating in conjunction with the limiting factor, mayovershadow the effect which ordinarily dom inates.

A somewhat similar state of afiairs may exist as regards the optimumdegree of vacuum in the oscillating tube. Allof the earlier experimentswith the device were made with as high a degree of vacuum as could besecured-much higher than that in the ordinary three element tube or inthe old style, gaseous discharge type of X-ray tube. Collisionionization phenomena within the tube are undesirable. Nevertheless,there is considerable evidence that small amounts of adsorbed gases onthe electrodes themselves are advantageous where high secondary emissionratios are desired, and it is therefore highly probable that operationof the device will be facilitated by the presence within the tube ofsmall quantities of inert gas, such as helium, neon, or others of thesame group, as long as the mean free path of electronswithin the tube islonger than the inter-electrode distances.

I claim:

1. An oscillation generator comprising an evacuated envelope, aplurality of electrodes within said envelope including an anode and acathode capable of emitting secondary electrons at a ratio to primaryelectrons greater than unity upon impact by such primary electrons, apotential source sufficient to accelerate electrons to a velocitycausing emission from said cathode at said ratio, means-includingcircuital connections between said electrodes for producing traversaland retraversal of the space between said electrodes by the secondaryelectrons emitted from said cathode to produce self-oscillation withinsaid circuital connections.

2. An oscillation generator comprising an evacuated envelope, aplurality of electrodes within said envelope including an anode and acathode capable of emitting secondary electrons at a ratio to primaryelectrons greater than unity upon impact by suchprimary electrons, apotential source sufficient to accelerate electrons to a velocitycausing emission from said cathode at said ratio, and means for'applying said potential to said electrodes, and means for cyclicallymodifying said potential by current flow to cause a cloud of electronsin a space path therebetween to oscillate back and forth along said pathand periodically to impact said cathode with sufficient velocity tomaintain said cloud of electrons at undiminished density independentlyof accretions'thereto by other than secondary electrons.

3. An oscillation generator comprising an evacuated envelope, aplurality of electrodes within said envelope including an anode and acathode capable of emitting secondary electrons at 'a ratio to primaryelectrons greater than unity upon impact by such primary electrons, apotential source sumcient to accelerate electrons to a velocity causingemission from said cathode at said ratio, and means including a resonantcircuit for modifying the potential from said source to cause theelectron cloud developed by the impactingprimary electrons to traverseand retraverse the space between said electrodes in response to theoscillating potential developed in said resonant circuit by the movementof said electron cloud.

4. An oscillation generator comprising an evacuated envelope containinga pair of opposed cathodes having surfaces adapted to emit electrons byimpact and an anode positioned therebetween, means for impartingvelocities to electrons emitted from said cathodes toward the opposedcathodes past said anode, an impedance connected to cause secondaryelectronsliberated from one of said cathodes to flow therethrough, andmeans for applying the voltage drop caused by electron flow in saidimpedance to impart sufficient additional velocity to the liberatedsecondary electrons to cause release of additional secondary electronsby impact with the other cathode.

5. An oscillation generator comprising a pair of opposed surfacescapable of emitting by impact a number of secondary electrons greaterthan the number of impacting primary electrons, an anode positioned tocollect a portion of said secondary electrons, means for guiding aportion of said secondary electrons at least as great as the number ofprimary electrons from either cathode toward the other in avoidance ofsaid anode, a current source for supplying the de ficiency of electronscaused by the secondary emission from said cathodes, and circuitalconnections including said cathodes and said source for applying avoltage drop caused by the flow of secondary'electrons-from one of saidcathodes to accelerate said electrons toward the other cathode to avelocity sufficient to cause secondary emission of electrons therefromat a ratio greater than unity.

.6. An oscillation generator comprising a pair of opposed cathodescapable of emitting secondary electrons by impact in excess of thenumber of impacting primary electrons, a source of potential greatenough to impart sufficient electron velocity to cause emission ofsecondaryelectrons in said amount of impact with said cathodes, andmeans including said source of potential and a resonant circuit forapplying potential from said source to cause electrons liberated byimpact from said cathodes alternately to impact the opposing cathode.

7. An oscillation generator comprising a pair of opposed cathodesadapted to emit secondary electrons by impact, an anode positionedbetween said cathodes, means for guiding a portion of secondaryelectrons emitted from either of said cathodes past said anode towardthe other cathode, a parallel resonant circuit connecting said cathodes,and a source of potential connected to apply a voltage between saidcathodes and said anode great enough to accelerate electrons to avelocity suiiicient to cause emission of secondary electrons from saidcathodes at .a ratio greater than unity.

. 8. An oscillation generator comprising an evacuated envelope, a pairof opposed cathodes positioned within said envelope, an anode sopositioned between said cathodes as to permit the greater portion of aspace current passing between said cathodes to pass uncollected by saidanode, a parallel resonant circuit connecting said cathodes, and asource of potential connected to i apply a voltage between said cathodesand said anode great enough to accelerate electrons to a velocitysuilicient to cause emission of secondary electrons from said cathodesat a ratio greater than unity.

9. An oscillation generator comprising an evacuated envelope, a pair ofopposed cathodes positioned within said envelope and having surfaces ofphotoelectric material, an anode so positioned between said cathodes asto permit the greater portion of a space current passing between saidcathodes to pass uncollected by said anode, a parallel resonant circuitconnecting said cath odes, and a source of potential connected to applya voltage between said cathodes and said anode great enough toaccelerate electrons to a velocity sumcient to cause emission ofsecondary electrons from said cathodes at a ratio greate than unity. I

10. An oscillation generator comprising a pair of opposed cathodesadapted to emit secondary electrons by impact, an anode positionedbetween said cathodes, means for guiding a portion of secondaryelectrons emitted from either of said cathodes past said anode towardthe other cathode, a parallel resonant circuit connecting said cathodes,and a source of potential sufliciently great to excite secondaryemission from said cathodes at a-ratio greater than unity connectedbetween said anode and a point intermediate the terminals of saidparallel resonant circuit.

11. An oscillation generator comprising an evacuated envelope containinga pair of opposed cathodes and an annular anode surrounding the paththerebetween, means for establishing a magnetic field longitudinal ofsaid path to guide electrons therealong between said cathodes, a sourceof potential suflicient to excite secondary emission from said cathodesat a ratio greater than unity, and circuital connections between saidanode, cathodes and source for applying a voltage drop resulting fromemission from either of said cathodes in a direction to accelerate theemitted electrons toward the other cathode.

12. An oscillation generator comprising an evacuated envelope, a pair ofopposed cathodes within said enevelope spaced to provide a free electronpath therebetween, a cooperating anode positioned to collect electronsdeviating from said path, means for guiding electrons from either ofsaid cathodes along said path, a source of potential great -enough tocause secondary emission of electrons from said 'cathodes at a ratio toimpacting electrons greater than unity, and circuital connectionsbetween said cathodes, anode electrons from said cathodes at a ratio toimpact-' ing electrons greater than unity, and a resonant circuitconnecting said cathodes, anode and source and tuned in relation to thetime of flight .of electrons between said cathodes as determined by thepotential of said source to apply a resonant potential to increase thevelocity of impact of electrons following said path.

14. An oscillation generator comprising an evacuated envelope, a pair ofopposed cathodes within said envelope spaced to provide a free electronpath therebetween, a cooperating anode positioned to collect electronsdeviating from said path, means for guiding electrons from either ofsaid cathodes along said path, a source of potential great enough tocause secondary emission of electrons from said cathodes at aratio toimpacting electrons greater than unity connected to said anode, and aparallel resonant circuit whose half period is of the same order ofmagnitude as the time of flight of electrons along said path under theinfluence of said source connected between said cathodes and having aconnection intermediate its ends to said source.

15. An oscillation generator comprising an evacuated envelope, a pair ofopposed cathodes within said envelope spaced to provide a free electronpath therebetween, a cooperating anode positioned to collect electronsdeviating from said path, means for guiding electrons from either ofsaid cathodes along said path, a source of potential great enough tocause secondary emission of electrons from said cathodes at a ratio toimpacting electrons greater than unity, and resonant connections betweensaid cathodes, anode and source for applying a voltage drop resultingfrom a fiow of electrons from one of said cathodes and supplied by saidsource to accelerate said electrons to a velocity of impact with theopposing cathode sufiicient to excite secondary emission therefrom atgreater than unity ratio.

16. An oscillation generator comprising an evacuated envelope, a pair ofparallel opposed cathodes spaced within said envelope and defining anelectron path therebetween, an annular anode surrounding said path, asource of potential sufficiently great to impart an electron velocitywhich will excite secondary emission from said cathodes at a ratiogreater than unity connected to said anode, and a resonant circuit tunedto a frequency whose half period is of the order of magnitude of thetime of flight of an electron between said cathodes under the influenceof said potential source connected between said cathodes and having anintermediate I terminal connected through said source to said anode.

17. An oscillation generator comprising an evacuated envelope, a pair ofparallel opposed cathodes spaced within said envelope and defining anelectron path therebetween, an anode grid intermediate said cathodescomprising an electrostatic shield of small actual area, to permit thepassage therethrough of a relatively large number of infallingelectrons, a source of potential suflicient to cause secondary emissionof electrons from said cathodes at a ratio greater than unity connectedto said anode, a connection between said cathodes, and an oscillatingcircuit connecting said anode and source and said cathodes tuned to afrequency whose period is of the order of magnitude of the time offlight of an electron between said cathodes under the influence of thepotential of said source.

18. The method of generating self-sustaining oscillations by means ofsecondary emitting surcausing a flow of current consequent to saidemission to produce a voltage drop, and applying said voltage drop toaccelerate said electrons to a like impact with another such surface.

19. The method of generating electrical oscillations which comprisesforming a cloud of electrons, oscillating said cloud along a path byapplying accelerating potentials thereto, maintaining said cloud bycausing the release of secondary electrons by impact at the end of saidpath, and deriving the potentials for causing said impacts from thecurrents comprising said seco'ndaryelectrons.

20. The method of generating electrical oscillations which comprisesforming a cloud of electrons, oscillating said cloud along a path byapplying accelerating potentials thereto, withdrawing a portion oftheelectrons from said cloud at each oscillation thereof to supplyuseful power,- causing said cloud to generate by impacts at the ends ofsaid path suflicient electrons to maintain said cloud and supply thosewithdrawn therefrom, and deriving the potentials for causing saidimpacts from the currents comprising said.

secondary electrons.

21. An oscillation generator comprising a pair of opposed cathodes,means for causing a cloud of electrons to oscillate between saidcathodes and impact thereon with velocity sufiicient to maintain thedensity of said cloud by the secondary electrons emitted by saidimpacts, and means for deriving from the current flow comprising saidsecondary electrons the energy required to.

give to said cloud the impacting velocity.

22. An oscillation generator comprising an evacuated envelope containingan anode and cathode structure including opposed surface portionscapable of emitting secondary electrons at a ratio to impacting primaryelectrons greater than unity, means including said anode for directingthe fiow of a portion of the electrons from one of said surfaceportions. toward the other without impacting said anode, means forcausing a current flow resultant upon the emission of electrons fromsaid cathode structure'to produce a potential on said anode so phasedinrelation to the time of emission of said electrons as to cause them toimpact said cathode structure with sufficient velocity to cause theemission of further secondary electrons.

23. An oscillation generator comprising an evacuated envelope containingan anode and a cathode structure capable of emitting secondary electronsat a ratio to impacting primary electrons greater than unity, meansincluding said anode for directing electrons emitted by said cathodestructure in a path avoiding said anode and returning to said cathodestructure, and means connecting said cathode structure and said anodefor applying potentials due to current flow resultant upon emission ofelectrons from said cathode structure to said electrons during theirtraversal of said path to impart thereto a final velocity sufiicient tocause'them to impact said cathode structure and release furthersecondary electrons thereby. 1

24. An oscillation generator comprising an evacuated envelope containingan anode and cathode structure, means including said anodeg fordirecting electrons liberated from said cathode in paths terminating insaid cathode struc: ture, and means for causing electrons traversingsaid paths to develop potentials between said anode and said cathodestructure sufficient to cause impact of said electrons with saidstructure causing emission of a greater number of secondary electronsthan impacting primary electrons.

25. Themethod of generating self-sustaining oscillations which comprisesthe steps of generating a cloud of electrons, subjecting said elec tronsto an electrostatic field directed towards a common center ofoscillation from a plurality of directions whereby electrons componentof said cloud released on one side of said center will tend to oscillateabout said center, withdrawing energy from certain of said oscillatingelectrons to damp the oscillations thereof to bring them to rest at saidcenter of oscillation, applying a portion of the energy thus withdrawnto other electrons component of the cloud to increase their net energy,and utilizing the additional energy of said last named electrons tocause them to release further electrons within said field.

26. The method of generating self-sustaining oscillations whichcomprises the steps of establishing an electrostatic field releasingelectrons in said field, causing said electrons to oscillate aboutacenter of oscillation within said field, causing the oscillation ofsaid electrons to so modify said field as to transfer energy fromcertain of said electrons to others, and using the) excess energytransferred to electrons whose energy is increased to release furtherelectrons in said field.

27. The method of generating self-sustaining oscillations by means ofsecondary electron-emitting surfaces, which comprises the steps ofgenerating an electrostatic field between a pair of such surfacesdirected toward a common center, releasing electrons within said fieldto oscillate about said center generating an oscillating potential fromthe motion of said electrons, and applying said potential to modify saidfield in such phase as to cause certain of said electrons to impact saidsurfaces with sufficient energy to release additional electrons withinsaid field.

PHILO T. FARNSWOR'II-l.

